Crash issue in PG18.5 regression

Hi,

We found that the program crashes when following the steps below.

1) Create the instance
initdb -D data --encoding=UTF8 --no-locale

2) Execute following SQL
SELECT to_date('01 ŞUB 2010', 'DD TMMON YYYY');

We are analyzing the cause and the following commit seems the cause.
https://github.com/postgres/postgres/commit/011384ba45f

Could you please check this?

FYI)
- This did not occur in PG17. We suspect that this is because PG17 is using `str_toupper()`.
- Build option is as below :
--enable-nls --with-libedit-preferred --with-openssl --with-krb-srvnam=postgres --with-gssapi --with-ldap --with-libcurl --with-libnuma --with-ossp-uuid --with-libxml --with-libxslt --with-perl --with-python --with-tcl --with-tclconfig=/usr/lib64 --with-pam --with-lz4 --with-zstd --enable-tap-tests --with-selinux TCLSH=/usr/bin/tclsh 'CC=gcc ' CFLAGS=-O2 'CPPFLAGS=-DLINUX_OOM_SCORE_ADJ=0 -DLINUX_OOM_ADJ' 'LDFLAGS= -Wl,-rpath,'\''$$ORIGIN/../lib'\'',--enable-new-dtags' --with-llvm LLVM_CONFIG=/usr/bin/llvm-config CLANG=/usr/bin/clang

Regards,
Masashi Kamura
Fujitsu Limited